1. Field of the Invention
This invention relates to the field of electron optics, and in particular to electron optics components for a semiconductor wafer and mask defect inspection system.
2. Description of the Related Art
Electron optical systems employed for imaging purposes typically generate a xe2x80x9cprimaryxe2x80x9d electron beam which is focused onto the surface of a substrate by probe-forming electron optics. The imaging process typically involves the collection of xe2x80x9csecondaryxe2x80x9d electrons which are emitted from the substrate surface as a result of the interaction of the primary electron beam with the substrate surface. In electron optical imaging systems, the energy of the primary electron beam is generally at least several hundred eV, while the secondary electron energies are predominantly below 10 eV. In order to form an image of the substrate, it is necessary to separate the secondary electrons from the primary electrons and collect these secondary electrons with some type of detector. In many electron optical systems, the secondary electron detector is positioned within the probe-forming optics, and a crossed magnetic-electric field filter (commonly called a xe2x80x9cWienxe2x80x9d filter) is used to deflect the secondary electrons off-axis into a detector, while simultaneously allowing the primary electrons (which are moving in the opposite direction), to pass through the Wien filter undeflected. In other electron optical systems, the secondary electron detector is positioned below the probe-forming optics, and off to one side of the electron optical axis of the probe-forming optics. In these systems, it is necessary that the electric fields from the secondary electron detector do not substantially affect the primary electron beam. This requirement typically limits the secondary electron collection efficiencies of these imaging systems. In systems where the primary electron beam energy is low, such as electron beam inspection systems, achieving high secondary electron collection efficiencies is particularly demanding. Clearly, there is a need for improved detector electron optics.
This invention includes an electron beam column incorporating an asymmetrical detector optics assembly. According to aspects of the invention, the electron beam column comprises: a probe optics assembly, for forming an electron probe; an electron optical axis, defined by the probe optics assembly; a secondary electron detector situated below the probe optics assembly; and a detector optics assembly, asymmetrical with respect to the electron optical axis, situated below the probe optics assembly. In preferred embodiments, the detector optics assembly comprises a field-free tube, asymmetrical with respect to the electron optical axis, situated between the probe optics assembly and the secondary electron detectors; the asymmetry can be introduced by offsetting the field-free tube from the electron optical axis or by chamfering the end of the tube. In some embodiments, the field-free tube has a circular bore and a square perimeter. In some embodiments the detector optics assembly comprises a field-free tube and a voltage contrast plate, either or both of which are asymmetrical with respect to the electron optical axis; the voltage contrast plate is situated below the field free tube and the secondary electron detectors. Asymmetry can be introduced for the voltage contrast plate by offsetting the plate from the electron optical axis, or by introducing notches or bumps onto the aperture in the plate. Some embodiments may include components of the detector optics assembly in which asymmetry has been introduced in multiple ways. The electron beam column may further comprise a stage situated below the detector optics assembly. The probe optics assembly comprises: an electron gun; an accelerating region situated below the electron gun; scanning deflectors situated below the accelerating region; and focusing lenses situated below the deflectors.